The cryogenic separation of air is a well established industrial process. Cryogenic air separation involves the filtering of the feed air to remove particulate matter and compression of that clean air to supply the energy required for the separation. Following the air compression, the feed air stream is cooled and cleaned of the high boiling contaminants, such as carbon dioxide and water vapor, and then separated into its components by cryogenic distillation. The separation columns are operated at cryogenic temperatures to allow the gas and liquid contacting necessary for separation by distillation, and the separated products are then returned to ambient temperature conditions versus the cooling air stream.
When argon recovery is desired in addition to separation of the air into nitrogen and oxygen, a commonly used system is one employing three columns wherein the air is separated into nitrogen and oxygen in the first two columns, a higher pressure and lower pressure column, which are generally in heat exchange relation at a main condenser, and wherein an argon containing stream is passed from the lower pressure column into a third column for production of crude argon. A discussion of this conventional process is found in R. E. Latimer, "Distillation of Air", Chemical Engineering Progress, Volume 63, pages 35-59 (1967).
The conventional three component air separation process is generally suitable for many purposes but has a significant disadvantage if nitrogen recovery is desired at elevated pressure. Of the three primary components of air, nitrogen is the most volatile, argon has intermediate volatility and oxygen is the least volatile. In order to enable high recovery of the individual components, the lower pressure column is operated at as low a pressure as possible, generally about 2 pounds per square inch (psi) above atmospheric pressure. This low pressure enables the relative volatilities between argon and oxygen and between nitrogen and argon to be as large as possible thus maximizing the separation of the air into the three components.
If nitrogen at moderate pressure is desired, the lower pressure column could be operated at a pressure above the conventional low pressure. However, this would result in a significant decrease in argon recovery because a significant amount of the argon would exit the process with the nitrogen rather than being passed to the crude argon column. Moderate pressure nitrogen is becoming in increasingly greater demand for such uses as blanketing, stirring, and enhanced oil recovery. Furthermore, production of moderate pressure nitrogen in conjunction with argon is increasing in importance as oxygen-argon air separation plants, originally built for the steel industry, are experiencing reduced utilization.
It is therefore very desirable to have an air separation process which can produce nitrogen at higher than conventional pressures while also enabling high argon recovery.
Accordingly, it is an object of this invention to provide an air separation process and apparatus for producing moderate pressure nitrogen while producing argon with high recovery.